ATHENS, Greece — A rocket was fired at the U.S. Embassy early Friday, striking the front of the building but causing no injuries.

Police cordoned off streets around the heavily guarded building after the explosion shortly before 6 a.m. The shell struck the third floor and smashed glass in nearby buildings.

Investigators found the device used to fire the rocket shell at a construction site near the embassy.

“This is an act of terrorism. We don’t know where from,” Attica Police Chief Asimakis Golfis said. “There was a shell that exploded in the toilets of the building. … It was fired from street level.”

Embassy officials confirmed that an explosion had taken place and said that no one had been injured. U.S. ambassador Charles Reis said the damage was “not extensive.”

“There can be no justification for such a senseless act of violence … The embassy was occupied at the time (but) nobody was hurt, he said.

Authorities were searching apartment buildings near the U.S. Embassy and a nearby hospital for evidence.

“I heard a loud bang. I didn’t realize what was going on,” said Giorgos Yiannoulis, who runs a kiosk near the embassy.

Traffic came to a standstill across parts of central Athens, as police and emergency services scrambled to the embassy building, which is a frequent destination for protest groups.

It was the first major attack against a U.S. target in Greece in more than a decade, following the arrest of members of the leftist November 17 terrorist group. The group was blamed for killing 23 people — including U.S., British and Turkish officials — and dozens of bomb attacks.

In 2003, a special court gave multiple life sentences to November 17’s leader, chief assassin and three other members. Lesser sentences were given to 10 others.